Birds See More Colors Than They Wear

Photonics.comJun 2011
NEW HAVEN, Conn., June 27, 2011 — The brilliant colors of birds have inspired poets and nature lovers, but researchers at Yale University and the University of Cambridge in the UK say that these existing hues represent only a fraction of what birds can actually see.

Findings based on study of the avian visual system, reported in the June 23 issue of the journal Behavioral Ecology, show that plumage colors went from dull to bright over millions of years as birds gradually evolved the ability to create newer pigments and structural colors.

“Our clothes were pretty drab before the invention of aniline dyes, but then color became cheap, and there was an explosion in the colorful clothes we wear today,” said Richard Prum of Yale. “The same type of thing seemed to have happened with birds.”

Birds are well-known for their stunning plumage colors, but new research reveals that they can see more colors than they can create. (Photo: iStockphoto)
Scientists have speculated for years on how birds obtained their colors, but the Yale/Cambridge study was the first to ask what the diversity of bird colors actually look like to birds themselves. Ironically, the answer is that birds see many more colors than humans can, but birds are also capable of seeing many more colors than they have in their plumage. Birds have additional color cones in their retina that are sensitive to ultraviolet range, so they see colors that are invisible to humans.

Prum and co-author Mary Caswell Stoddard of Cambridge measured hundreds of plumage colors and analyzed them in a tetrachromatic color space, which combines raw information about the light that feathers reflect with details about the colors that birds can see. They found that bird plumage colors fall far short of filling the color space, leaving vast regions unoccupied.

“Just as a newspaper can only print a fraction of the colors we humans can see, bird feathers can only produce a subset of colors that are theoretically visible to other birds,” Stoddard said. “The intriguing part is thinking about why plumage colors are confined to this subset. Out-of-gamut colors may be impossible to make with available mechanisms, or they may be disadvantageous.”

Over time, birds have evolved a dazzling combination of colors that include various melanin pigments, which give human skin its tint; carotenoid pigments, which come from their diets; and structural colors, such as the blue eyes of humans. The study shows that the structural colors produce the majority of color diversity to bird feathers, even though they are relatively rare among birds.

“Birds can make only about 26 to 30 percent of the colors they are capable of seeing, but they have been working hard over millions of years to overcome these limitations,” Prum said. “The startling thing to realize is that, although the colors of birds look so incredibly diverse and beautiful to us, we are colorblind compared to birds.”